1. Field of the Invention
This invention relates to dampening water feed rollers such as a master water roller and a water spreading roller which are used in a continuous water feed device in a planographic printing press.
The planographic printing is a process of printing which utilizes the nature of an oily ink to repel water and uses a printing plate having an oleophilic picture formed on a hydrophilic base. The printing is effected by indirect printing of the so-called offset printing technique which comprises alternately feeding water and ink to the plate surface, causing the ink selectively adhering to the picture portion of the plate surface to be transferred tentatively to a blanket, and transferring the ink from the blanket to an object of printing. The printing press to be used for the planographic printing, therefore, is inherently provided with a dampening mechanism for feeding water to the non-picture portion of the printing plate and an inking mechanism for feeding ink to the picture portion.
A notable example of the conventional dampening mechanism is configured as follows.
This dampening mechanism is provided with a metallic master water roller which is rotated while it is partly submerged in a water supply pan disposed underneath, together with a metering roller of rubber and a water applying roller of rubber which are both held in contact with the master water roller. In this mechanism, the water adhering to the surface of the master water roller and consequently ascending with the rotation of this roller is deprived of an excess portion thereof in consequence of its contact with the metering roller. The remaining water, owing to the contact of the master water roller and the water applying roller, is transferred to the water applying roller. The water applying roller, on contact with the plate cylinder, passes the water to the non-picture portion of the surface of plate cylinder. The water which remains after failure to adhere to the surface of the plate cylinder is returned from the plate cylinder back to the water supply pan through the medium of the intervening rollers.
Another dampening mechanism comprises a metallic water discharging roller which is rotated while it is partly submerged in a water supply pan installed underneath, a water transferring roller of rubber held in contact with the water discharging roller, a metallic water spreading roller held in contact with the water transferring roller, and a water applying roller of rubber held in contact with the water spreading roller. In this dampening water mechanism, the water adhering to the surface of the water discharging roller and consequently ascending from the water supply pan with the rotation of the water discharging roller is deprived of an excess portion thereof during the course of transfer through the medium of the water discharging roller and the water spreading roller, then guided to the water applying roller, and passed from the water applying roller to the non-picture portion of the surfaces of the plate cylinder with which the water applying roller comes into contact. The water which remains after failure to reach the picture portion of the surface of the plate cylinder is returned from the plate cylinder back to the water supply pan through the medium of the intervening rollers.
As described above, the dampening mechanism, owing to the alternate arrangement of rubber rollers and metallic rollers, is enabled to feed the water in the water supply pan to the non-picture portion of the plate cylinder and return the water remaining after failure to reach the picture portion back to the water supply pan through the medium of the intervening rollers.
2. Description of the Prior Art
It has been heretofore customary for the metallic rollers used in the dampening mechanism of the planographic printing press to have their surfaces coated with a hard chromium plating to acquire improved resistance to corrosion and abrasion and enhanced affinity for water.
In spite of the hard chromium plating of the nature described above, such metallic rollers as the master water roller and the water spreading roller which are used in the dampening mechanism are still deficient in hydrophilicity and water-retaining property and, therefore, entail the following difficulties in printing. The water film on these rollers draws into drops or loss uniformity and impairs the uniformity of density of the ink film on the plate cylinder; the uniformity of supply of the dampening water is disrupted and, as the result, the so-called halftone of the picture portion is deprived of definition; and the dampening water is not fed sufficiently to the non-picture portion of the plate and, as the result, the ink adheres to the non-picture portion and smears the plate surface.
Further, when the water which remains after failure to reach the picture portion of the plate cylinder is returned to the water supply pan through the medium of the intervening rollers as described above, the ink on the picture portion of the plate cylinder is partly entrained by the water applying roller and passed in the form of emulsified ink to the surfaces of the rollers. Since these metallic rollers are deficient in hydrophilicity, they do not manifest the action of repelling the emulsified ink sufficiently and consequently suffer from fast deposition of the emulsified ink. Once the ink is deposited fast, the supply of the dampening water to the non-picture portion of the plate can no longer be continued. Thus, the non-picture portion of the plate is smeared with the adhering ink. For the plate cylinder to provide fine printing at all times, therefore, the printing press must be stopped periodically to permit cleaning of the defiled plate surface.
From this point of view, the idea of incorporating isopropyl alcohol or other similar alcohol and a surfactant in the dampening water thereby lowering the water surface tension and enhancing the water's ability to wet the metallic rollers has found acceptance. The incorporation of such substances as alcohol and surfactant in the dampening water, however, adds to cost and jeopardizes the hygienic condition of the workshop environment and causes the undesirable phenomenon of swelling in the rubber rollers which are exposed to the dampening water.
Japanese Utility Model Publication SHO 55(1980)-14,518 discloses a water spreading roller having a porous layer formed on the surface of a steel pipe or stainless steel pipe by the flame spraying of ceramic, and Japanese Utility Model Unexamined Publication SHO 62(1987)-116,869 discloses a master water roller having a layer of an oxide type ceramic formed by plasma flame spraying. These rollers merely have layers formed on metallic pipes by the flame spraying of ceramic. These layers are claimed to possess a porous surface. As recited in Japanese Utility Model Publication SHO 55(1980)-14,518, the porosity of surface is aimed at enhancing the rollers' water-retaining property. The enhancement of the water-retaining property due to the porous ceramic coating, however, is excessive for the master water roller or the water spreading roller. The water is suffered to ascend overly to the plate cylinder and the ink is rendered susceptible to emulsification. Since the master water roller and the water spreading roller are rotated as held in contact with the water applying roller, the ink adhering to the surface of the water applying roller permeates into the pores of the ceramic coating on the rollers and manifests an action of repelling water and impairs the hydrophilicity required of these rollers. Thus, the stable supply of the dampening water can be no longer be attained unless the dampening water incorporates therein an alcohol.
Japanese Utility Model Unexamined Publication SHO 62(1987)-116,868 discloses a dampening roller provided with a coating layer of an oxide type ceramic composed of 40 to 80% by weight of Cr.sub.2 O.sub.3, 10 to 30% by weight of Al.sub.2 O.sub.3, and 10 to 30% by weight of SiO.sub.2, and Japanese Utility Model Unexamined Publication SHO 62(1987)-136,353 discloses a dampening roller having a layer of an oxide type ceramic formed by plasma flame spraying and having the porous part of the flame sprayed layer occluded with an oxide type ceramic coating agent composed of 40 to 80% by weight of Cr.sub.2 O.sub.3, 10 to 30% by weight of Al.sub.2 O.sub.3, and 10 to 30% by weight of SiO.sub.2. These dampening rollers have a surface layer formed of a compact oxide complex ceramic material using Cr.sub.2 O.sub.3 as a matrix or a surface layer formed of an oxide ceramic material by flame spraying and additionally have the pores in the surface layer occluded with a coating agent, so as to acquire enhanced hydrophilicity owing to the use of a ceramic layer on the surface. The Cr.sub.2 O.sub.3 ceramic substance has no conspicuous effect in improving the rollers wettability with water. Even by the use of rollers of such a structure, the dampening mechanism cannot be operated satisfactorily when the dampening water incorporates absolutely no alcohol therein. Moreover, the oxide composite ceramic layer of compact texture using Cr.sub.2 O.sub.3 as a matrix is expensive because the production thereof requires the immersion in the Cr.sub.2 O.sub.3 slurry and the heating to 400.degree. to 500.degree. C. to be alternately repeated several to ten or more times.